To reduce discharge of fuel vapors into the atmosphere, motor vehicles induct fuel vapors from a fuel tank into the engine. A carbon canister is also coupled to the fuel tank to absorb fuel vapors under some conditions when the internal combustion engine is not running. For example, a carbon canister may adsorb refueling, diurnal and running loss vapors. The carbon canister, however, has limited capacity thus may be periodically purged. To purge the canister, engine running manifold vacuum may be used to desorb the vapor from the activated carbon canister via opening of a canister purge valve (CPV). Desorbed vapors are combusted in the engine.
In some applications, plug-in hybrid electric vehicles (PHEV) may include hydrocarbon (HC) traps in an air induction system of the engine to adsorb fuel vapors in the engine intake, e.g., fuel leaked from fuel injectors and/or fuel collected in the intake, in order to further reduce emissions. To purge fuel vapor stored in an HC trap in the engine intake, a throttle plate in the engine intake may be opened during engine operation to cause air flow which desorbs the HC from the trap. Desorbed vapors are combusted in engine.
In the case of plug-in hybrid vehicles, the internal combustion engine may not operate for a prolonged period of time. Since engine run-time is limited in these applications, purging of an HC trap in the engine intake may also be limited. For example, if HC traps are loaded with hydrocarbons from fuel vapors, the engine may have to be forced on to purge the traps. This results in a fuel economy efficiency penalty.
The inventors herein have recognized the above-described issues and, in one example approach, a method for a vehicle with an engine including an HC trap in an intake of the engine is provided. The method comprises, in response to an ambient temperature decrease during an engine off condition while a fuel tank is sealed from atmosphere, delivering fuel stored in a hydrocarbon trap in an intake of the engine to a fuel vapor canister coupled to the fuel tank in an emission control system.
In this way, vacuum generated in a fuel tank via naturally occurring diurnal temperature changes may be used to passively purge an HC trap in the engine intake while the engine is off, e.g., following a key-off event. The fuel vapors in the HC trap may be delivered to a fuel vapor canister with larger storage capacity for storage therein during engine off conditions. In hybrid vehicle applications, the fuel vapor canister is typically clean as it is designed to adsorb refueling vapors only and refueling events are infrequent. Thus the refueling canister may be used as an HC “storage bank” so that the HC trap may be clean out repeatedly until the engine kicks in to clean out the refueling canister. This results in reduced HC breakthrough in the HC trap leading to reduced emissions while increasing fuel economy.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.